The present invention relates to electrostatic printing apparatus, and more particularly to method and apparatus for transferring and fusing toned electrostatic images to plain paper or other receptor substrates.
Electrostatic imaging processes initially require the formation of a latent electrostatic image, typically by electrographic or electrophotographic means, on a dielectric member. The latent electrostatic image is subsequently toned, and the toned image may be transferred to a further member. In the usual case, the toned image is transferred to plain paper. The transferred image is usually subsequently fused or fixed to the receptor medium, to provide enhanced durability and permanence.
Various toner image transfer methods are known in the prior art. The transfer may be accomplished electrostatically, by means of a charge of opposite polarity to the charge on the toner particles, the former charge being used to draw the toner particles off the dielectric member and onto the image receptor.
Alternatively, the image receptor medium may be passed between the toned member and a transfer member, and the toner image transferred by means of pressure at the point of contact. Usually, the toner image is fused to the image receptor subsequently to transfer of the image, at a further process station. Post transfer fusing may be accomplished by pressure, radiation, or hot roll.
It is possible, however, to accomplish transfer and fusing of the image simultaneously. This may be accomplished by a heated roller or simply by means of high pressure between the image-bearing dielectric member and a transfer member, between which the image receptor passes. Apparatus utilizing high pressure for simultaneous transfer/fusing is disclosed in Fotland and Carrish, U.S. Pat. No. 4,365,549 (Dec. 28, 1982).
A problem that is typically encountered in transferring a toner image solely by means of pressure is the existence of a residual toner image on the dielectric member after image transfer, due to inefficiencies in toner transfer. These residual toner particles require scraper blades or other removal means, and can eventually accumulate at the various process stations associated with the dielectric member, including the apparatus for forming the latent electrostatic image. These toner accumulations decrease the reliability of the apparatus, necessitating service at intervals. Furthermore, such inefficiencies in toner transfer may lead to mottling of the images formed on the image receptor sheets.
Shepard and Meldurm, British Patent No. 1,271,606 (Apr. 19, 1972) disclose apparatus for transferring a toner image onto an image receptor, involving an imaging roller and a pressure roller arranged to be normally in contact under high pressure. Various embodiments are disclosed for achieving a slippage between the surface of the imaging roller (from which the toner image is transferred) and the image receptor. These include covering the pressure roller with an elastic material, and driving the rollers to achieve a peripheral speed differential. The apparatus is said to achieve lower transfer pressure requirements and provide higher toner transfer efficiency.
Beaudet, U.S. Pat. No. 4,894,687 (Jan. 16, 1990) teaches the use of skewed rollers to improve the efficiency of toner transfer. As a receptor web moves through a skewed nip, the web moves with lateral slip thus generating a shearing strain at the web-toner interface. This shear, along with relative paper-image roll motion, increases the transfer efficiency of the system.
Skewing of the rollers, however, leads to significant problems relating to web handling. A transverse force is imparted to a paper web passing through the nip formed by skewed rollers. The web tends to move to the higher paper web tension side of the pressure roll assembly. Unless very high web tension is employed, this can result in wrinkling of the web. It is very difficult to control the lateral position of the web. This lack of control results in color registration problems in multi-station printer systems.
The transferred image is skewed. A rectangle, for example, printed on the imaging roller appears as a parallelogram after transfer. The printed image must, therefore, be printed in a reverse skew to compensate the transfer skew.
The motion of the paper through the pressure nip is controlled by the relative coefficients of friction between the paper/image roll and the paper/transfer roll. The coefficient of friction between the paper and image roll varies with time due to the presence of toner between the image roll surface and paper. In general, the presence of toner reduces the apparent coefficient of friction between paper and image roll. This results in a lateral displacement of the web between as the toned area on the image roll varies.
Accordingly, it is an objective of the invention to provide improved electrostatic imaging apparatus for pressure transfer of a toner image from a dielectric surface to a receptor sheet that overcomes limitations associated with web tracking errors. A related object of the invention is to provide a transfer method which effects simultaneous fusing of the toner image.
Another objective of the present invention is the reduction in the skew requirement to obtain high efficiency of toner transfer.
Another objective of the invention is simultaneous pressure transfer/fusing characterized by a high efficiency of toner transfer.
A further objective is to provide a method and apparatus that minimizes paper distortion and wrinkling under high nip pressures.
Another objective of the invention is to provide a constant paper motion thus simplifying multi-stage image registration.
It is a further objective of the invention to minimize the amount of residual toner on the dielectric image member after transfer/fusing. Related objects are increased reliability of electrostatic imaging apparatus, and decreased service requirements.
A further, related objective of the invention is to provide apparatus that allows the creation of high quality toned images at high speeds.
In furthering the above and additional objectives, the invention provides method and apparatus for simultaneously transferring an image from an imaging roll to a receptor surface in a nip formed between the imaging roll and a transfer roll using shear strain between the imaging roll and receptor sheet to provide high transfer efficiency. The improvement involves clamping the receptor sheet to the transfer roll so that all of the shear strain appears between the imaging roll and the receptor surface. Clamping may be effected by providing a third pressure roll to form a second nip between the transfer roll and the pressure roll. The combination of high friction in the second nip and the imaging surface wrap around the pressure roll effectively cause the imaging surface to accurately track the motion of the transfer roll.
High transfer efficiency is provided by either the presence of skew between the image and transfer roll, as in the case of U.S. Pat. No. 4,894,687, or through the use of differential speed as described in British Patent No. 1,271,606.